1. Field of the Invention
The present invention relates to a crystallization method, a method of manufacturing a thin film transistor, a thin film transistor, a display, and a semiconductor device.
2. Description of the Related Art
Because a thin film transistor (referred to as TFT) can be formed on surfaces of various materials, a circuit for display can be directly formed, for example, on an inner wall surface of a display panel within a display. An early TFT directly forms a channel region on an amorphous silicon film on a substrate. A current TFT forms a channel region on a polycrystalline silicon film with an amorphous silicon film crystallized. In the current TFT, carrier mobility is ten times faster than that of the early TFT.
On the above polycrystalline silicon film, crystals with the grain size of about 0.1 μm are scattered irregularly. In the TFT with a channel region formed on the polycrystalline silicon film, the channel region includes a lot of crystal grain boundaries, which results in unevenness in properties. The crystal grain boundary presents resistance to an electron moving within the channel region or a hole.
The inventors aim to obtain the same transistor property as that of a transistor formed on a silicon wafer, propose a crystallization method for forming an array of large crystal grains enough to form one or a plurality of TFTs within one crystal grain, in Surface Science Vol. 21, No. 5, P. 278-287 (2000) (Non-Patent Document 1), and develop its industrialization technique. Forming a TFT within one crystal grain has less ill effect on the transistor property caused by the grain boundaries and improves the TFT property dramatically, unlike the conventional transistor in which a grain boundary is formed within a channel region.
A driving circuit of a display such as a liquid crystal display has been formed so far separately from the amorphous semiconductor film formed on a glass substrate that is a display panel. With an increase in the IT market, information to be handled is digitalized and speeded up, together with a requirement for higher image quality in a display. As means for satisfying the above requirement for higher image quality, a switching transistor which switches each pixel is formed on the crystal semiconductor. Therefore, switching speed gets faster with higher image quality.
In the display, there are a signal processor such as a DA converter which converts digital image data into analog image signal and a gate array which processes digital image data, and a driving circuit such as a signal line drive circuit and a scanning line drive circuit, in addition to a pixel switching circuit. There is a demand to integrate these circuits onto the same display panel substrate together with the pixel switching circuit. In order to satisfy the demand, it is necessary to develop a TFT with higher carrier mobility and less unevenness in the mobility and threshold voltage property.